In a turbine engine compressor, the variable-pitch nozzle stages are mounted downstream or upstream from rotor wheels of compression stages. The variable-pitch vanes in each nozzle stage are carried by the annular casing and they are adjustable in angular position about their respective axes in order to optimize the flow of gas through the core of the turbine engine.
Each variable-pitch vane has a substantially cylindrical axial pivot at each of its ends, these pivots defining the pivot axis of the vane. The radially outer pivot is engaged in a substantially radial chimney of the casing and it is connected by a lever to a control ring that extends around the casing and that is generally actuated by an actuator. A turning movement of the control ring about the longitudinal axis of the casing is transmitted by the levers to the outer pivots of the vanes, thereby causing the vanes to pivot about their axes.
In operation, the control ring tends to deform or to come out of alignment with the longitudinal axis of the casing, and possibly also to be offset from a radial plane. In order to limit or prevent this phenomenon, it is known to fit the control ring with runners for bearing against an outer cylindrical track of the casing, with the control ring extending around the track. The runners of the control ring co-operate with the track of the casing to keep the ring centered in spite of the off-centering forces that result from the thrust of the actuator and the reactions of the levers connected to the vanes.
In the prior art, the casing is made of a metal alloy and its control ring centering track(s) is/are made integrally with the casing in the form of local annular extra thicknesses of the casing. Each of these extra thicknesses serves solely to create a centering track and it does not have any function of reinforcing or stiffening a zone of the casing. Nevertheless, the extra thickness leads to a significant increase in the weight of the casing. Furthermore, the cylindrical track is made by milling, thereby increasing the time required for fabricating the casing, and thus increasing its cost.
It should also be understood that the tilting behavior of the vanes, when worn, is improved by the vanes having outer pivots of a radial size that is large, however that causes the control ring to be further away from the casing and thus increases the extra thickness constituted by the track on the casing. In other words, if it is desired to improve the behavior of the vanes in operation by increasing the radial size of their outer pivots, that leads automatically to a significant increase in the outside diameter of the track and thus to an increase in the weight of the casing.